Snap-on beads are used in prior art windows as a very convenient way for retaining a windowpane. The beads are removable parts interlocked with the framework for exerting a holding pressure on the periphery of the windowpane, more specifically the periphery which is on the side opposite the rabbet of the framework. The beads comprise a polymeric glazing material, also called a weatherstrip or wedge gasket, that is in direct contact with the windowpane.
The framework holds the windowpane in a wall opening or a similar location. It may be made of various materials such as aluminum, polyvinyl chloride (PVC), fiberglass, wood, etc. The windowpane is usually adjusted in the framework with shims, such as neoprene setting blocks, and pushed against the glazing sealing tape. The glazing sealing tape is previously set in the rabbet and is used to deter rain penetration. The beads then lock the windowpane with a proper compression between their weatherstrips and the glazing sealing tape.
In high performance windows, a heel bead of gunnable acrylic sealant is installed once the windowpane is in place. The heel bead is installed by hand all around in the space between the perimeter of the windowpane and the framework for providing an air seal. The heel bead is used in conjunction with a backing rod previously installed by hand in the space.
Installation of the heel bead is a very critical step because if it is too thin, it would not be suitably efficient and if too deeply installed, it may contact the sealant of the factory-sealed multiple glazing unit, if any. Since many sealants are chemically incompatible, very important problems may arise if the sealant of such unit deteriorates, thereby possibly leading to the penetration of humidity and condensation between the multiple glazings. In the latter case, the sealed unit will have to be replaced by the contractor or the owner at his own expenses because the warranty from the unit manufacturers exclude claims of this ground.
Great care must be taken when installing a heel bead, but quality is often sacrificed to speed up the process because the installation is time-consuming and tedious for the worker and costly for the contractor. As a result, the heel bead is often imperfect and it will be hard to detect the flaws once the snap-on beads are in position in the framework.
Additionally, although backing rods are available in different sizes and shapes, the same set of backing rods is often used all around the windowpane in spite of the fact that the space between the perimeter of the windowpane and the framework is not always equal on all sides. As a result, the backing rod is not positioned at the optimum deepness and the heel bead will not be suitable.
The presence of the shims is also a problem when installing a heel bead because it has to be often deviated or disjoined around the shims, increasing the possibility of flaws.
Another drawback with the use of a heel bead arises whenever the windowpane has to be replaced, after a break thereof for instance. If this happens, the old heel bead must be completely removed all around the framework so that the surface be fully clean and prepared for the new heel bead, otherwise the new heel bead would not be satisfactory. The surface cleaning is often a costly, time-consuming and tedious process, as it is for the installation of the new heel bead itself.
A further drawback with the use of a heel bead is that it does not significantly insulate the region between the periphery of the windowpane and the front flange of the bead, which is particularly notable in the case of an aluminum bead and framework. It is due to the direct contact of the front flange with the part of the framework adjacent the perimeter of the windowpane. This results in important heat losses through the beads and to possible condensation on the beads during cold weather, especially on the lower horizontal bead. The heat losses are even greater if the heel bead is defective.